During the assembly of semiconductor packages, semiconductor chips are often attached onto carriers, such as substrates or leadframes. After or during attachment, electrical connections are made between electrical pads on the chips to corresponding contacts or connection pads on the substrates or leadframes. This can be done by wire bonding, or the electrical pads can be directly attached onto the contacts on the substrates or leadframes. Thereafter, it is usually necessary to protect the chips and the electrical connections by encapsulating them in an encapsulation compound, such as epoxy molding compound (“EMC”).
In a typical transfer molding process, the substrate or leadframe with the chips attached is placed into a molding system comprising two mold halves. One or more molding cavities are formed in one or both of the mold halves corresponding to the positions of the chips to be encapsulated. Molding compound is introduced into mold supply pots in the molding system in pellet form. The mold supply pots are linked to the cavities through a system of runners and gates through which the molding compound is channeled before entering the cavities. A plunger is insertable into each pot under heat and pressure to crush the pellet and distribute molding compound under the pressure from the plunger through the system of runners and gates and into the molding cavities.
After the cavities have been filled, the molding compound is allowed to set. Besides molding compound that is filled into the cavities, excess molding compound is also created adjacent to the mold supply pot, and in the runners and gates. FIG. 1 is an isometric view of hardened molding compound created using a transfer molding system of the prior art, including a so-called cull portion 102. A leadframe 100 has been molded with molding compound. The hardened molding compound comprises a cull portion 102, a runner portion 104, a gate portion 106 and an encapsulation portion 108 covering the leadframe 100. The encapsulation portion 108 protects internal components (not shown) on the leadframe 100 and would be retained. The cull portion 102, runner portion 104 and gate portion 106 are not used and are removed and discarded before further processing of the leadframe 100.
Of the compound to be discarded, the cull compound 102 created is significant. Therefore, it represents significant wasted molding compound, and it is desirable that the cull compound 102 should be reduced or eliminated altogether to save costs.
FIG. 2 is a plan view of a mold chase 110 of the prior art. The mold chase 110 has recesses corresponding to the various portions of the hardened molding compound as described above. The mold chase 110 has so-called culls 112 corresponding to the cull portion 102 that is formed. The culls 112 are located adjacent to mold supply pots into which molding compound in pellet form is placed for distribution of molding compound. The culls 112 are linked via runners 114 to gates 116 leading to molding cavities 118. Molding compound is thus transferred from the culls 112 to the runners 114 and the gates 116 before being introduced into the molding cavities 118. The runners 114 correspond to the runner portion 104, the gates 116 correspond to the gate portion 106 and the molding cavities 118 correspond to the encapsulation portion 108.
This problem of cull is, for example, addressed in U.S. Pat. No. 5,520,874 entitled “Compressible Mold Plunger”. It describes a plunger that has a movable piston and spring that reduces the cull space when the piston is pressed downward into the cull mold compound. It thus seeks to reduce the cull formed during molding. However, it cannot totally eliminate cull because there is still a substantial amount of molding compound that remains between the plunger and a recess in the mold chase corresponding to the position of the mold supply pot even after the plunger applies maximum pressure. The resulting hardened molding compound would still look substantially similar to that referred to in FIG. 1, as it has a large cull area. It would be advantageous to further reduce the amount of cull or eliminate cull produced by such molding process without compromising the effectiveness of the molding process.